The Deadman Control
Do you use the phrase ‘deadman’ or ‘deadman switch’ when talking about safety-related controls on your machinery? I often run into this when I’m working with clients who use the terms to refer to ‘enabling devices’ – you know, those two or three-position switches that are found on robot teaching pendants and in other applications to give the operator a way to stop machinery, even if they have already been injured or killed by the equipment. Calling these devices a ‘Deadman Switch’ or even a ‘Live-Man Switch’ as the three-position devices are sometimes called, sends entirely the wrong message to the user as far as I’m concerned. The objective of our work as machinery safety engineers is to prevent injuries from happening in the first place. Using a device that is designed to determine if the user is dead or unconscious means someone screwed up!
A little history
The term ‘deadman’ comes from a device that was developed in the 1880’s by pioneering electrical engineer Frank Sprague. Sprague was working on electric traction motor technology, using these new machines to power street railways (streetcars) and electric elevators. The early DC motor controls used in both streetcars and elevators required an operator. The operator used a hand control to move the streetcar forward or backward along the track and to control the speed of the car. In elevators, the operator used a similar hand control to move the elevator car up or down the shaft, to control the speed and to stop at the appropriate floor.
If the operator was to doze off or fall unconscious, the streetcar would simply continue on its way until it hit something or derailed, either being a poor option! Elevators would continue until they hit the top or bottom of the shaft, again a bad idea. Sprague included a control device in his designs that required the operator to keep his hand on the controller handle and to maintain pressure on the control device in order for power to flow to the motor. This same idea was implemented in manual elevator control handles. These ideas were adopted by Westinghouse when they developed the streetcar motor controllers that were used in thousands of streetcars running between the 1890’s and the 1930’s.
When diesel-electric and full electric locomotives were developed in the 1930’s, the concept of the ‘deadman’ control was adopted from street railways. There is a persistent myth that these controls started with steam locomotives. In fact, an earlier version of this article included that myth – now busted!
With the advent of electric trams, trains, and subways, concerns about possibilities like heart attacks and other infirmities resulting in drivers losing control of these machines caused these devices to be integrated into these new transportation systems. To learn more about these applications, see the Wikipedia article Dead Man’s Switch.
It’s worth noting that the railways now call these devices ‘Driver Safety Devices’ or DSD. See a modern DSD at the Arrowvale Electronics web site.
Elevators moved from manual control to automatic control, eliminating the need for elevator operators and the need for ‘deadman’ controls.
Robots Enter the Picture
In the 1980’s, industrial robots began to appear in the workplace. Accidents in these early days drove changes in the design of the control pendants used to ‘teach’ these devices their tasks. Early pendants provided motion control and an emergency stop device. Later, the motion controls were altered to become ‘hold-to-run’ devices that could jog the selected robot axis at a pre-selected slow-speed, one axis at a time. In the 90’s the ‘enabling device’ was added to the pendant. These two-position switches, still called ‘dead-man switches’, had to be held closed in order for the robot to move under control of the axis hold-to-run controls. Accidents continued to occur. In the mid 90’s the three-position enabling device, sometimes called a ‘live-man-switch’, was introduced after studies showed that some people would release their grip on the control pendant when struck by the robot, while others would clench the hand holding the pendant. The new switches are required to be held in the mid position to enable motion. The picture at left shows the back of a modern robot pendant. The black bar in the lower right is the enabling device, located so that your hand will naturally hold the device in the correct position when you hold the pendant in your left hand. Not so good if you are left-handed!
In addition to the pendant enabling devices, additional enabling devices are required where more than one worker is required inside the danger zone of the machine. These devices can be purchased separately and added to systems as needed. Depending on the application, you can get these devices with emergency stop buttons and jog buttons integrated into a single unit as shown in the picture of the Euchner ZS switches.
Machinery Standards and Definitions
The enabling device is one of those protective measures that cannot be readily classified as a safeguarding device because they do not proactively prevent injury. Instead, like an emergency stop, they may allow a worker to avert or limit the harm that is already occurring. That places the enabling device into the ‘complementary protective measure’ category.
Let’s take a minute to look at a couple of important definitions from the machinery standards. At the moment, the best definition for a complementary protective measure comes from the Canadian standard, CSA Z432-04. Excerpted from CSA Z432-04, §126.96.36.199.3 Complementary Protective Measures:
Protective measures that are neither inherently safe design measures, nor safeguarding (implementation of guards and/or protective devices), nor information for use may have to be implemented as required by the intended use and the reasonably foreseeable misuse of the machine. Such measures shall include, but not be limited to,
a) emergency stop;
b) means of rescue of trapped persons; and
c) means of energy isolation and dissipation.
Let’s also look at the formal definition of an ‘enabling device’ in the same standard:
7.23.3 Enabling devices
An enabling device is an additional manually operated 2- or 3-position control device used in conjunction with a start control and which, when continuously actuated in one position only, allows a machine to function. In any other position, motion is stopped or a start is prevented.
Enabling devices shall have the following features:
a) They shall be connected to a Category 0 or a Category 1 stop (see NFPA 79).
b) They shall be designed in accordance with ergonomic principles:
(i) position 1 is the off function of the switch (actuator is not operated);
(ii) position 2 is the enabling function (actuator is operated); and
(iii) position 3 (if used) is the off function of the switch (actuator is not operated past its mid position).
c) Three-position enabling devices shall be designed to require manual operation in order to reach position 3.
d) When returning from position 3 to position 2, the function shall not be enabled.
e) An enabling device shall automatically return to its off function when its actuator is not manually held in the enabling position.
Note: Tests have shown that human reaction to an emergency may be to release an object or to hold on tighter, thus compressing an enabling device. The ergonomic issues of sustained activation should be considered during design and installation of the enabling device.
Similar definitions exist in the International, European and US standards, although they may not be quite as formalized.
Most enabling devices on their own do nothing except PERMIT motion to take place, although the actual definition of enabling device in CSA Z432-04 actually permits the enabling device to cause motion. Absence of the enabling signal prevents or stops motion. These devices are then used in conjunction with hold-to-run controls on robots and machinery, and with throttle controls on trains, street cars, subways and similar equipment. Note that most standards to not permit enabling devices to actually cause motion. This is a unique situation in the Canadian standard.
So what’s the big deal?
Using the terms ‘dead-man’ or ‘live-man’ to describe these devices puts the wrong message out as far as I’m concerned. As safety engineers and OHS practitioners, we care about keeping workers out of danger. This is neither checking to see if we have either a ‘dead man’ or a ‘live man’, but rather ensuring that the person in control of the equipment is ‘in control’. Using a phrase like ‘enabling device’ clearly says what the device does.
In my opinion, and supported by the current International and Canadian Standards, these terms must be abandoned in favour of ‘enabling device’ and the qualifiers ‘2-position enabling device’ and ‘3-position enabling device’. These terms are also used in many of the current machinery safety standards, so using them correctly improves clarity in writing and speaking. Clarity in communication in safety is too important for practitioners to permit the ongoing use of terms that convey the wrong message and do not promote clarity of meaning. Since clarity is often lacking when it comes to safety, anything we can do to improve our communications should be high on our priority list!
Ed. note: This post was updated on 17-Aug-17. The myth of deadman controls on steam locomotives was removed and replaced by historically verifiable information about the origins of this control.